Middle East Technical University
SSME -518
Assignment 1
Preparing a Printed Material
Submitted by
: Harika Özge Kızılsu
Submitted to
: Aysegül Eryılmaz
Due Date
: 30.11.2004
: Enzymes
Intended audience
: 9 Th grade high school biology students
Required entry capabilities : Students should know some chemical concepts
such as chemical reaction, activation energy. They should know the previous
biology subjects which have been instructed from the beginning of the term like,
proteins. They should interpret the graphics.
Explain the functions of enzymes in living cells.
Define the terms “catalyst” and “substrate”.
Describe the lock and key model of enzyme action.
Define the term “activation energy”.
Explain the effects of temperature, PH and enzymes and substrate
concentrations on enzyme action.
 The importance of enzymes
 How enzyme work
 Activation energy
 Factors effecting enzyme activity
 Temperature
 pH
 Enzyme concentration
 Substrate concentration
 Bill Indge, Mike Boyle, AQA (A) Biology: Molecules, Cells and Systems
(Collins Student Support Materials S.), 2000 p: 57-72
 Bill Indge, Margaret Baker, Martin Rowland A New Introduction to Human
Biology (AQA Specification A. S.), 2002 P: 68-72
 http://bio.winona.msus.edu/bates/genbio/chapter4.htm
The Importance of Enzymes
Enzymes are protein substances that are
necessary for most of the chemical reactions that
occur in living cells. There is a major difference
between chemical reactions in nonliving things
and the reactions in living things. Consider, for
example, the burning of gasoline in an
automobile engine. The gasoline vapor admitted
to the engine cylinder. It is ignited by a spark.
The vapor burns in a fraction of a second. In fact,
the burning is so rapid that it produces a small
explosion, which helps to drive the engine.
Enzymes make this possible in the living cell.
Enzymes enter into a chemical reaction only
temporarily, speed up the rate of chemical reactions
by as much as a million types and at the end of the
reaction they remain unchanged. They are used again
and again for the same chemical step with other
molecules. A substance that brings about a reaction
without being changed itself is called a catalyst.
Enzymes are amazing molecules. All enzymes in
a living organism are made by the cells of the
organism. Each cell contains several hundred
enzymes. Most of these enzymes are used within
the cell in which they are made. However, some
enzymes are passed out of the cell to catalyze
reactions outside the cell. All the digestive
enzymes that are produced in the human
digestive track are of this type. For example,
pepsin is an enzyme made inside the cell of
glands in the stomach wall. It leaves the cell and
breaks down proteins into simpler molecules.
How Enzymes Work
The ability of enzymes to act as catalysts depends on
their shape. Each different enzyme has its own shape,
with a “pocket” at a particular position. The pocket is
called the active side. The substrate molecules fit
the shape of the active site (Figure1).
The substance that an enzyme acts upon is called its
substrate. When the substrate molecule comes in contact
with the active side of the enzyme, it forms a temporary
union with the enzyme. This is called an enzyme-substrate
complex. During this time the enzyme may break bonds
within the substrate molecule. The substrate is separated
into 2 smaller molecules which are called products.
Break down
of substrate
Enzyme ready
to repeat
Figure1: Lock and Key Model; the substrate fits
exactly into the active site on the enzyme.
This process can be explained like this:
E + S
P + E
There is a theory to explain how the enzyme and
substrate fit together at an active site. It is called the
lock and key model. Just as the notched surface of a
key can open only one lock, the shape of the active
site of an enzyme only fits certain substrates. Thus,
each enzyme can catalyze a reaction only of those
substrates. They are extremely specific, generally
reacting with only one substrate.
Activation Energy
The energy required to make substrates react is
called the activation energy. This can be illustrated
in the energy diagram (Figure 2).
At the start of the reaction, starting substance has a
certain amount of energy. In rearranging the bonds
of starting substance some energy is released, so the
products have less energy than the initial molecules.
The minimum amount of energy needed to start the
reaction is the activation energy.
There are two ways to get the boulder to roll
down the hill. You can supply enough energy to
push it to the top of the mound, where it can then
roll down by itself. This is equivalent to
supplying heat to start a reaction.
Alternatively, you could dig away at the mount,
reducing the energy needed to push the boulder to
the top of the mound. This is equivalent to
supplying an enzyme to a reaction (Figure 3).
Instead of actually supplying energy, the enzyme reduces the
height of the energy barrier and therefore reduces the
activation energy necessary for a reaction to take place. This
may not seem very impressive but the involvement of an
enzyme increases the rate of the reaction by a billion times!!
Factors Affecting Enzyme Action
Figure2: Energy diagram of reaction.
Every chemical reaction thus has an energy barrier
that has to be overcome before a reaction can occur.
A comparison that is often used is that of a boulder
resting on top of a hill (Figure 2). Although it will
naturally roll down the hill the boulder is prevented
from doing so by a small mound of earth.
Experiments have shown that many factors can affect
the action of enzymes in living cells. These factors
affect only the rate of a catalyzed reaction. The
products formed by the reaction do not change.
Enzymes enable cell reactions to take place at
normal temperatures. Many chemical reactions that
take place slowly at ordinary temperatures can be
speeded up by raising temperatures. However, high
temperatures can kill living cells. Enzymes speed
up reactions without requiring high temperatures.
Enzyme action depends on the random motion of
molecules because this motion brings the substrates into
contact with the enzymes. The motion increases as the
temperature rises. If the temperature is low, the rate at
which enzyme-substrate complexes form will be low,
and the effect of the enzyme will be reduced (Figure4).
Rate of reaction
36 C (optimum
Figure 3: Energy diagram illustrating that
enzymes reduce the activation energy.
Figure 4: The Effect of Temperature on the Rate of
Enzyme Action.
If the temperature is reduced to near or below
freezing point, enzymes are inactivated, not
denatured. They will regain their function when
higher temperatures are restored.
The effectiveness of an enzyme depends on the PH
of the surrounding medium. Most enzymes function
efficiently over a narrow pH range. A change in pH
above or below this range reduces the rate of
enzyme activity considerably (Figure5).
Enzyme concentration
The rate of an enzyme-controlled reaction depends on
how often enzyme and substrate molecules bump into
each other. When there is little enzyme but a great deal
of substrate, the concentration of enzyme limits the rate
of the reaction. In this case, the total number of enzyme
molecules is acting only a small fraction of the
available substrate molecules. Adding more enzymes,
therefore, increases the number of substrate molecules
that can be reacting at any moment. Consequently, the
reaction rate increases until a maximum rate is reached,
as shown in Figure 6. The reaction rate reaches a
maximum when all substrate molecules are occupied
continuously. After this point, adding more enzymes
will not increase the rate of the reaction.
Rate of reaction
At higher temperatures, the enzyme becomes more
effective, because complexes are forming at a faster
rate. At still higher temperatures, however, the
enzyme itself starts to break down. This process is
called denaturation. When the shape of the enzyme
molecule changes, its active site no longer fits the
substrate molecule, and it loses its effectiveness.
There is a particular temperature-the optimum
temperature- at which an enzyme is most effective.
Optimum temperatures for enzymes in living cells are
usually close to the normal cell temperature (36C).
Substrate concentration
Rate of reaction
Enzym e concentration
Figure 6: The Effect of Enzyme Concentration on the
Rate of Enzyme Action
Figure 5: The Effect of pH on the Rate of
Enzyme Action
Changes in pH lead to the breaking of the enzyme
bounds. The enzyme begins to lose its functional
shape, particularly the shape of the active site. The
enzyme is said to be denatured.
The PH of the contents of the human stomach, for
example, is slightly acidic. Pepsin is most effective
at this pH level. The pH in the intestine is slightly
basic. At this pH, the enzyme trypsin, which
continues the digestion of proteins, works best.
Substrate concentration
The effect of substrate concentration is similar to the
effect of enzyme concentration. When there is less
substrate than enzyme, the amount of substrate limits
the reaction rate. Thus, adding more substrate allows
all the enzyme molecules to function simultaneously.
Therefore, the reaction rate increases until a maximum
rate is reached, as shown Figure 7. This rate is reached
because all the enzyme molecules are occupied.
Rate of reaction
Enzyme concentration
Substrate concentration
Optimum pH is the pH at which an enzyme
catalyses a reaction at the maximum rate.
Figure 7: The Effect of Substrate Concentration
on the Rate of Enzyme Action
The concentrations of enzyme or substrate
determine reaction rates as long as both are
freely accessible to each other. However,
enzyme and substrate may not be accessible to
each other when, for example, a membrane or
other structure separates them. In this case, the
rate at which the substrate crosses the membrane
or other barrier limits the rate of the reaction.
The time needed for an enzyme-substrate
complex to form and a reaction to occur
is very short. A single enzyme molecule
can catalyze thousands of substrate
reactions each second. Thus, only small
amounts of an enzyme need to be present
in a cell at any given time.
1. Enzymes are
catalysts that
the rate of chemical
reaction in living system.
2. In the lock and key model, the active site is
(flexible/ rigid).
5. Activation energy is:
A. Energy that must be added to get a reaction
started, which is recovered as the reaction
B. Difference in energy between reactants and
C. Energy that is lost as heat.
D. Free energy.
6. Which statement about enzyme catalyzed
reactions is NOT true?
A. Enzymes form complexes with their
B. Enzymes lower the activation energy for
chemical reactions.
C. Many enzymes change shape slightly when
substrate binds.
D. Reactions occur at the "active site" of
enzymes, where a precise 3D orientation of
amino acids is an important feature of
7. Explain what is meant by the key-lock model
3. What effect does an enzyme have on the rate of
the reaction?
4. Is an enzyme changed by the reaction?

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